The purpose of a railroad tie is to connect the earth, or other intermediate supporting base, to plates which connect to rails. They also provide for the proper spacing (gauge) between rails. In turn the rails support locomotives, passenger, freight or service cars as they transit or park.
FIG. 1 shows the cross section of a treated timber tie 10 in a common cross section of seven inches (7″) tall and nine inches (9″) wide. Common lengths for cross ties are eight feet (8′), eight foot and six inches (8′-6″) and nine feet (9′). Switch ties are longer. In this drawing the pressured applied preservative 20 does not penetrate through the entire tie. There is a core 30 that may remain untreated.
Railroad ties are traditionally made of wood, though some are of concrete or all-plastic or plastic-composite. There are several standard sizes, one common size being seven inches tall by nine inches wide by nine feet long. Other standards include cross sections of 6″×8″, 6″×9″ and lengths of 8′-0″ and 8′-6″.
Ties must be strong enough to maintain support and gauge under lateral loads, static vertical loads, and dynamic vertical loads. The tie must be resistant to the dynamic load which can cause the tie plate to move and abrade the tie. The tie must be able to function despite environmental stresses of thermal expansion, ultraviolet (UV) radiation, attack from microorganisms, fungi, insects and other life forms. It is highly preferable that ties be installable using the existing base of standardized installation equipment and fasteners. Some rail systems use a “third rail” to conduct power to trains. For this and other reasons, railroad ties should not be conductors of electricity.
The predominant tie in service is a hardwood timber treated with creosote, coal tar, chromated copper arsenate or other preservative. Over time these preservatives leach from the tie to the surrounding earth and eventually migrate to the surrounding areas, including water tables. There are few safe methods for disposing of treated timber ties. Stacking them in landfills does little to retard leaching. Open air burning releases the toxins into the atmosphere. Closed effluent burning with contaminant capture is expensive.
Because concrete and reinforced concrete ties are highly inflexible they do not allow a flex-and-resume support of the rails. More concrete ties are required per mile of track which increases the cost per mile. The cost per tie is also higher. Further, the increased weight of concrete requires changes to installation equipment and procedures.
Both timber and concrete ties can accept water into cracks or grain separations. As water freezes it expands and can force the cracks wider, leading to a reduction in tie strength. For reinforced concrete ties this crack expansion can also expose the metallic reinforcing material to air, thereby initiating the deleterious effects of rust, further reducing tie strength.
More than ten million ties were installed as new or replacements during each of 2003-2006. With thousands of ties per mile, the introduction of a functionally equivalent or superior, longer lived, and lower life cycle cost tie is materially beneficial to rail operators, maintains or improves rail system safety, and is ecologically beneficial.
Thus, there is a need for a tie with a combination of lower manufacturing times, better spike retention, increased resistance to abrasion, lighter weight, and lower cost than existing concrete, plastic or composite ties.
There is a further need for processes for manufacturing a tie having the above characteristics in an efficient and environmentally sensitive manner.